The present invention is directed to the calibration of data receivers, and is particularly concerned with the calibration of vector demodulators for receiving data transmitted by means of quadrature modulation format.
Demodulators for data that is transmitted with quadrature modulation, such as phase shift keyed (PSK) data, can introduce losses into the received data as a result of various conditions inherent in the demodulator. Examples of these conditions include gain imbalance in which the in-phase signal is amplified by a different amount than the quadrature signal, quadrature imbalance in which the phase difference between the two signals is not precisely 90.degree., DC offsets in either or both of the in-phase and quadrature channels and scale errors resulting from the net effect of gains and losses in the system. Each of these potential sources of signal loss must be measured to determine calibration factors. Once determined, these factors are used to adjust the system and transform the received data into actual data.
In the past, the determination of the calibration factors is made by transforming the received signal into the frequency domain and analyzing the power spectrum of the received signal. In this approach, each of the calibration factors are separately determined. Typically, the DC offsets of the system are first measured. Once the received signal is adjusted for these offsets, gain imbalance is determined. After adjustment for the gain imbalance, quadrature imbalance is determined. Unfortunately, these various factors are not independent of one another. If one is changed, the others may vary as well. As a result, after an initial determination has been made for each of these factors, they must be successively reevaluated and readjusted. Thus, it is not always possible to have each of the errors converge to zero. Typically, in such a case the calibration process is terminated once the quadrature error has been brought to zero, even if the other errors cannot be completely corrected.
Another drawback associated with the traditional approach of transforming the measurements into the frequency domain and determining calibration factors from the power spectrum relates to the fact that this approach requires two signals of known frequency with continuously varying phase. As such, common signal formats such as 8PSK do not provide sufficient information for calibration to be performed. However, the 8PSK mode of operation is preferred, since it results in an easier and faster generation of vector measurements.
Furthermore, spectral methods of the type traditionally employed to perform calibration measurements are highly sensitive to anomalies in system performance which may be due to random disturbances, e.g. accidental bumping of the equipment or temperature fluctuations. As a result, the calibration factors that are determined may be seriously biased.
It is desirable therefore to provide a method for calibrating data receivers, such as vector demodulators, which avoids these drawbacks. In particular, it is desirable to provide a calibration method that enables all calibration factors to be determined simultaneously, and thereby account for the interdependence of such factors. Along these lines, it is also desirable to provide a calibration technique which enables an 8PSK mode to be used as well as a continuously varying phase, and which can account for aberrations which may be introduced by random disturbances in the system.